This invention relates to imaging of etch resistant layers also known as xe2x80x9cresistsxe2x80x9d. Resists exposed according to the invention may be used to fabricate high resolution patterns by etching or deposition. The invention may be applied to the manufacture of integrated circuits, flat panel displays and printed circuit boards, for example.
Processes for fabricating high resolution patterns, mainly on planar objects, by selective etching or deposition are well known. In general, a layer to be shaped or patterned, which may be called a substrate, is covered by a protective layer known as a xe2x80x9cresistxe2x80x9d. In general, a resist is used as an imagewise mask for selectively controlling a chemical or physical process. The resist limits the process to follow an image pattern defined by the resist. The term xe2x80x9cresistxe2x80x9d should be interpreted in this broad sense throughout this disclosure and claims. Most commonly used resists operate by undergoing a change of solubility in a developer when they are exposed.
An image made up of desired shapes is created on the resist usually via photo-imaging. The exposed (or unexposed, if the resist is negative working) parts of the image are removed, normally by using a liquid developer to expose the substrate. The substrate can now be treated, for example by etching through the openings in the resist layer. The treatment is limited to areas of the substrate adjacent the openings. Portions of the substrate which remain covered by the resist are protected from the etching or other treatment.
Etching may be done, for example, by wet chemicals or by dry plasma (a process widely used in the semiconductor industry). Resists are also used in additive processes in which one or more materials are deposited through openings in the resist to add to the substrate. Deposition may be done in a wet process (as in the well known xe2x80x9cadditivexe2x80x9d process for manufacturing printed circuit boards) or in a dry process, such as a vacuum deposition by evaporation or sputtering or CVD. Resists may also be used to permit chemical reactions, such as oxidation, to occur only in selected areas of a substrate which are not covered by the resist.
At the end of the process the remaining resist is normally removed, or xe2x80x9cstrippedxe2x80x9d. Historically most resists were photoresists, i.e. activated and imaged by the photonic action of the light. Because of this photonic action most photoresists operate in the UV part of the spectrum, where the photon energy is high. Some resists can be exposed by other types of radiation, such as electron-beams. All photoresists and electron-beam resists share one fundamental property: they respond to the total exposure, not to the momentary illumination.
In optics, exposure is defined as the integral of illumination over time. When a certain exposure is reached, a change occurs in the resist. The change depends upon the exposure but not upon the intensity of light used to achieve that exposure. For example, a photoresist can be exposed by 100 mW/cm2 for 1 sec to yield an exposure of 100 mJ/cm2 (100 mWxc3x971 sec). The same exposure results when the photoresist is exposed by 1000 mW for 0.1 sec with similar results (1000 mWxc3x970.1 sec=100 mJ/cm2). This law, also known as the xe2x80x9creciprocity lawxe2x80x9d, is the basic law governing the exposure of photoresists.
The law of reciprocity requires that photoresists and other integrating resists be exposed with the use of an optical system which provides a high contrast ratio and low stray light. For example, if an exposure system has a leakage, or stray light, of 1% (e.g.: when exposure is xe2x80x9coffxe2x80x9d, the light level does not drop to zero but only drops to 1% of the xe2x80x9conxe2x80x9d state) the effect of this stray light may be as large (or larger) than the main exposure. The effects of stray light accumulate over time and are especially significant if the photoresist is exposed for a long time to the xe2x80x9coffxe2x80x9d state.
An even larger problem is caused when trying to image closely-spaced high resolution features: the point spread function of any practical optical system causes a xe2x80x9cspreadingxe2x80x9d of light from each feature. Stray light from one feature illuminates adjacent features and lowers the resolution. FIG. 1 illustrates this problem. A first feature 1 has a light distribution 1xe2x80x2 and a second feature 3 has a light distribution 3xe2x80x2. Exposure curve 2, generated by lens 8 imaging first feature 1, is added to exposure curve 4, generated by imaging second feature 3, to create a curve 5, which is the equivalent exposure. Curve 5 creates distorted images 6 and 7 of features 1 and 3 on photoresist 9 which has a threshold 10. It makes no difference whether exposures 2 and 4 are applied simultaneously or sequentially. The photoresist will add up, or integrate, the exposures.
The problems described above can be compounded if the surface of the resist is not flat. It is known in the art to treat the surfaces of semiconductors in various ways to enhance planarity. This can increase the cost of manufacturing semiconductor devices. FIG. 4 shows the what occurs when a prior art system is used to expose a non-planar substrate 12 coated with a photoresist 9. The deviation from planarity need not be large in order to cause a problem. When making integrated circuits, the depth of focus is typically below 1 micron due to the large numerical aperture of the lenses used. A deviation of 1 micron can be caused by a build-up of lower layers. Today a CMP process (Chemical-Mechanical Polishing) is used to bring the silicon wafer back to planarity. If lens 8 is focused on the substrate 12 at one point, all points higher or lower than the plane of focus will be out of focus causing loss of imaging resolution. For example, narrow lines will widen and merge (or narrow gaps will disappear). It is not possible to correct this problem by repeating the exposure at a different focus setting because, when the same substrate (which obeys the law of reciprocity) is imaged again at a different focus setting, all the exposure which was absorbed but did not reach the threshold will add up with the new exposure and destroy the image.
Recently a different type of resist, known as thermoresist, has been used in the manufacturing of printing plates and printed circuit boards. A thermoresist (also known as a thermal resist or heat-mode resist) changes solubility when a certain temperature, rather than a certain accumulated exposure, has been reached. Such thermoresists are imaged using near infra-red light and therefore are also known as xe2x80x9cIR resistsxe2x80x9d. Some exampled of thermoresists are disclosed in U.S. Pat. No. 5,340,699 (Haley); U.S. Pat. No. 5,372,907 (Haley); U.S. Pat. No. 5,372,915 (Haley); U.S. Pat. No. 5,466,557 (Haley); U.S. Pat. No. 5,512,418 (Ma); U.S. Pat. No. 5,641,608 (Grunwald); U.S. Pat. No. 5,182,188 (Cole); U.S. Pat. No. 5,314,785 (Vogel) and U.S. Pat. No. 5,328,811 (Brestel). The thermoresist described by Haley is unusual as the same composition acts as a photoresist, obeying the reciprocity law, when exposed by UV light (at low power densities) but also acts as a thermoresist, responding only to temperature, when heated up by infrared light at high power densities. Thermal resist is also available from Creo Ltd. (Lod Industrial Park, Israel), sold under the trade name xe2x80x9cDifine 4LFxe2x80x9d. All of the above mentioned thermoresists respond to temperature and do not follow the reciprocity law. Such resists may be called xe2x80x9cnon-integratingxe2x80x9d. It is not possible to have a practical true thermoresist which follows the reciprocity law. Such a thermoresist would become exposed simply by long exposure to ambient temperature (just as a photoresist can be exposed by a long exposure to low levels of ambient light). While it is possible to shield a photoresist from ambient light it is not possible to shield from ambient temperature. Therefore a practical thermoresist cannot obey the reciprocity law.
Prolonged exposures to ambient temperatures below the threshold temperature has little effect on a thermoresist. Obviously, the threshold temperature needs to be well above the temperatures expected to be encountered in shipping and storage. When the chemical reaction in a thermoresist does not have a sharp threshold temperature, the chemical composition is formulated to keep the reaction rate very low at room temperature. This is not difficult to do, as most chemical reaction rates approximately double every 10 degrees centigrade. Thus the reaction rate in a thermoresist exposed at 350 degrees centigrade can be a billion times faster than at 25 degrees. Using lasers it is fairly easy to raise the temperature of a thermoresist to over 1000 degrees. Such a thermoresist will appear to have a distinct threshold simply because the reaction rate at lower temperature slows down exponentially. To follow the reciprocity law the reaction rate would have to change in a linear fashion with temperature.
Light valves, also known as multi-channel modulators or spatial light modulators, break up a single light beam into a linear or two-dimensional array of individually addressable spots. Examples of devices which use light valves to expose photoresists are shown in U.S. Pat. No. 5,208,818 (Gelbart) and U.S. Pat. No. 5,296,891 (Vogt). The limiting factor in both these patents is the leakage light from the light valves used. Even if the light valves were ideal, the limited optical resolution of the imaging lens creates a problem equivalent to stray light as previously explained. Multi-beam, also known as multi-spot, scanning is well known in the art and is used to increase writing speed by exposing a plurality of features simultaneously.
There is a need for methods for imaging resists on non-planar substrates. There is a particular need for such methods which can provide high resolution imaging and for such methods which do not require high contrast optical systems.
This invention takes advantage of the fact that a non-integrating resists, such as thermoresists are not substantially affected by exposure to light or other radiation at levels insufficient to expose the resist. The invention uses this property to image non-planar resists in multiple exposures such that, in each exposure, only xe2x80x9cin-focusxe2x80x9d parts of the resist are imaged.
One aspect of the invention provides a method for imagewise exposing a non-planar resist layer. The method comprises providing a variable focus optical system and a non-planar, layer of a non-integrating resist on a substrate. The resist may be a thermoresist. The method illuminates selected areas on a surface of the resist layer a first time with the optical system at a first focus setting and thereby causes parts of the resist which are within the selected areas on the substrate and are at a first elevation to be converted from an unexposed state to an exposed state. The resist in the selected areas at a second elevation where the image is not in focus are not converted to the exposed state. The method also illuminates the selected areas on the surface of the resist layer a second time with the optical system at a second focus setting and thereby causes parts of the resist which are within the selected areas on the substrate and are at the second elevation to be converted from the unexposed state to the exposed state.
In preferred embodiments, there is a delay which is longer than a thermal time constant of the resist layer between illuminating the surface the first and second times.
Even when a thermoresist is used, the illumination can be provided at ultraviolet wavelengths. This permits high resolution imaging of the resist. The optical system may, for example, comprise an ultraviolet laser light source.
In preferred embodiments the surface of the resist is illuminated a plurality of times with the optical system at the first focus setting. A different set of selected areas is exposed in each of the plurality of times. For example, in some embodiments an image to be exposed on the resist layer comprises a plurality of pixels arranged in a grid comprising a plurality of rows and a plurality of columns. An image to be exposed on the resist comprises a selected set of the pixels. Illuminating the surface of the resist the first time comprises separately illuminating two or more groups of the selected pixels such that in the groups of the selected pixels no two pixels are in adjacent rows and no two pixels are in adjacent columns. This prevents stray light from one pixel from interfering with the proper exposure of adjacent pixels.
Another aspect of the invention comprises a method for imagewise exposing a non-planar resist layer. The method comprises providing a variable focus optical system and a non-planar, layer of a non-integrating thermoresist on a substrate, the thermoresist changing from an unexposed state to an exposed state upon heating to a threshold temperature; focusing the optical system to generate an in-focus image of a set of features in a first plane of focus of the optical system; illuminating the set of features on a surface of the resist layer a first time for a duration sufficient to heat the illuminated portions of the resist which are in the first plane of focus to a temperature in excess of the threshold temperature at an intensity such that illuminated portions of the resist which are not in the first plane of focus are not heated to the threshold temperature; focusing the optical system to generate an in-focus image of a set of features in a second plane of focus of the optical system; and, illuminating the set of features on a surface of the resist layer a second time for a duration sufficient to heat the illuminated portions of the resist which are in the second plane of focus to a temperature in excess of the threshold temperature at an intensity such that illuminated portions of the resist which are not in the second plane of focus are not heated to the threshold temperature.
The methods of the invention have particular application in the fields of manufacturing of integrated circuits, circuit boards and displays. Further features and advantages of the invention are described below.